Needle coke is used as an aggregate for a graphite electrode used in electric furnace steel making processes and is generally produced using petroleum-based heavy oil or coal tar as the raw material. In a process of producing a graphite electrode, coke particles and a binder pitch are blended at a predetermined ratio, and then kneaded while being heated, and extrusion-molded thereby producing a green electrode. The green electrode is calcined to be graphitized and fabricated thereby producing a graphite electrode product.
The graphite electrode is desirously lower in coefficient of thermal expansion (CTF) because it is used under severe conditions such as high temperature conditions. That is, a graphite electrode with a lower CTF is less consumed and thus can reduce the cost of the electric furnace steel making.
The above-mentioned graphitization is a process wherein a green electrode is heated at a temperature of about 3000° C. and a direct current flow furnace (LWG furnace) is generally used. However, graphitization carried out in the LWG furnace accelerates the temperature elevating rate therein and thus facilitates the generation of gas. As the result, an abnormal expansion phenomenon, so-called puffing is likely to occur. Puffing lowers the density of an electrode and also sometimes breaks the electrode. However, the accelerated temperature elevating rate has been demanded with the objective of reducing costs, and there is a strong demand for needle coke with higher strength, lower expansion rate and lower puffing characteristics so that it can withstand such an accelerated temperature elevating rate.
Now, a method has been studied wherein coefficient of thermal expansion and puffing characteristics are controlled upon production of needle coke, and there have been proposed various methods. For example, Patent Document 1 discloses a method wherein a coal tar pitch from which quinoline-insolubles has been substantially removed is blended with an oligomer adjusted in polymerization degree and coked by the delayed coking method. Patent Document 2 discloses a method wherein a coal tar-based heavy oil and a petroleum-based heavy oil are blended at a specific ratio such that the nitrogen and sulfur contents are to be 1.0 percent by mass or less and 1.4 percent by mass or less, respectively to prepare a feedstock which is then placed into a delayed coker to produce a green coke, which is then calcined at a temperature of 700 to 900° C. and cooled, and again calcined at a temperature of 1200 to 1600° C. Patent Document 3 discloses a method wherein upon production of coal tar by rapid thermal cracking of coal, the thermal cracking temperature in the reactor is kept at 750° C. or higher and the residence time of the thermal cracked product in the reactor is 5 seconds or shorter thereby producing a liquid product which or the pitch of which is then carbonized. Patent Document 4 discloses a method wherein needle coke is produced by subjecting a petroleum-based heavy oil alone or a mixture thereof with a coal tar-based heavy oil from which quinoline-insolubles have been removed, as the feedstock to delayed coking and thereupon the petroleum-based heavy oil has been so adjusted that the content of particles such as ash therein is to be from 0.05 to 1 percent by mass.                (Patent Document 1) Japanese Patent Laid-Open Publication No. 5-105881        (Patent Document 2) Japanese Patent Laid-Open Publication No. 5-163491        (Patent Document 3) Japanese Patent Laid-Open Publication No. 5-202362        (Patent Document 4) Japanese Patent Laid-Open Publication No. 7-3267        